This invention relates generally to low pass filters, and more specifically concerns such filters wherein more than one form of filtering is necessary.
While R.C. active filters wherein emitter followers form the active devices have been widely used, a problem arises when more than one form of filtering becomes necessary. For example, if a four-pole filter is needed in one application, but a six-pole filter is required in another similar application (as for example in filter duty applications in multi-channel telephony), it becomes necessary to employ multiple active devices such as transistors (for example) in order to prevent device variations from adversely affecting performance. However, each transistor added to the active filter necessitates the employment of a different biasing point, which is a distinct disadvantage. Accordingly, there is a need for a high performance filter wherein multiple active stages are employed but which avoids the multiple biasing point difficulty.